DRY up a common view pattern

Define a Whisper.View base class that automatically parses and renders
templates and attributes defined by the subclass. This saves us a good
number of lines of code as well as some marginal memory overhead, since
we are no longer saving per-instance copies of template strings.

js/views/toast_view.js

@@ -17,10 +17,9 @@ var Whisper = Whisper || {};
 
 (function () {
     'use strict';
-    Whisper.ToastView = Backbone.View.extend({
+    Whisper.ToastView = Whisper.View.extend({
         className: 'toast',
         initialize: function() {
-            Mustache.parse(this.template);
             this.$el.hide();
         },